Non-ferroelectric, non-chiral, liquid crystals have been utilized as electro-optic materials in a number of applications requiring low drive voltages, large apertures, low cost and compact structures. These include watch, calculator and television displays, spatial light modulators, real-time holographic media, switches and tunable filters. These liquid crystal devices operate on the basic principle that due to the dielectric anisotropy of nematic, cholesteric and smectic liquid crystals, the average molecular axis, or director, can be oriented in the presence of an applied electric field. The coupling of non-ferroelectric liquid crystals to the applied field is a weak, second order interaction. In general, slow response times are characteristic of non-ferroelectric, non-chiral, liquid crystal optical devices.
Meyer et al. ("Ferroelectric Liquid Crystals", in Le Journal de Physique, Vol. 36, March, 1975, pp. L69-L71) showed that chiral C* or H*, smectic liquid crystals, could be ferroelectric, that is, possess a permanent electric dipole density, P. This permanent polarization, P, is perpendicular to the average orientation of the long axis of the molecules (denoted by the molecular director, n,) and generally contained within a plane parallel to the smectic layers. In these chiral smectic liquid crystals (CSLCs), the molecular director makes a temperature dependent angle, .PSI., with respect to the layer normal, z, as shown in FIGS. 1 and 2. In general, .PSI. ranges from 0.degree. to 45.degree.. The presence of the electric dipole provides a much stronger coupling to the applied electric field as compared to non-ferroelectric liquid crystals. Furthermore, the coupling, and hence aligning torque is about linear with applied field. The significance of this is that changing the sign of the applied electric field will change the direction of P in smectic C*, H*, A* and other chiral smectic phase liquid crystals. While n is constrained to an angle, .PSI., relative to z, the chiral nature of these materials causes n to rotate by angle .phi. about the layer normal, forming a large number of degenerate energy states oriented along a cone in bulk CSLC samples. The molecular directors are free to rotate by .phi. from layer to layer, and the cone structure forms a helix along the z direction. Therefore, even though chiral smectic liquid crystals possess microscopic permanent polarization, they cancel in the bulk in the planar-aligned orientation, producing little if any macroscopic polarization, or ferroelectric properties.
N. A. Clark et al. in U.S. Pat. No. 4,367,924, realized a ferroelectric liquid crystal switching device by sandwiching a thin layer of a smectic C* (SmC*) liquid crystal between two glass plates coated with transparent electrodes. In this patent, they describe the surface-stabilized ferroelectric liquid crystal (SSFLC) device, which employs SmC* or SmH* liquid crystal phases in the so-called bookshelf geometry, otherwise designated the planar alignment, where the smectic layers are perpendicular to and the liquid crystal molecules are parallel to the glass plates which also contain the electrodes, as illustrated in FIG. 1. Surface stabilization suppresses the helix, and allows the molecular directors to align in one of two states, .+-..PSI. to the layer normal. In the chiral smectic C,F,G,H,I liquid crystalline phases, a planar-aligned, surface-stabilized cell results in a binary, intensity switching device (see also N. A. Clark et al. U.S. Pat. No. 4,563,059 and N. A. Clark and S. T. Lagerwall in Applied Phys. Letts. (1980) 36:899 and S. T. Lagerwall and I. Dahl Mol. Cryst. Liq. Cryst. (1984) 114:151-187). The application of the electric field, thus, selects between the discrete orientations (two for SmC* materials) of the optic axis of the cell, each of which represents a different optical transmission states of the cell. The voltage required for switching such cells are modest (.+-.10V), and power consumption is quite low because the FLC switching energy required is small. SSFLC SmC* materials have been shown to be useful in a number of electro-optic device applications including switches, shutters, displays and spatial light modulators (SLM's). The materials most commonly employed have .PSI. of .+-.22.5.degree. (.pi./8) and switch between two stable states by a rotation of the optic axis of 2.PSI. or 45.degree. (.pi./4). Typically in a SSFLC light switch, SmC* layer thickness is selected to provide a retardation of .pi. (i.e., a half-wave) at a design wavelength. In this case, input polarization, which is aligned with the optic axis of the SmC* material in one of its states, is rotated by 45.degree. (.pi./4), i.e., two times an angle between the polarized input and the optic axis of the crystal (by a reflection about the fast axis of the crystal) when the field across the cell is reversed. Such a device therefore functions as a binary, intensity modulator. The advantages of these chiral smectic C,F,G,H, and I liquid crystal devices is their nearly three orders of magnitude increase in switching speeds over non-chiral liquid crystal devices and their intrinsic bistability, which has applications for optical memory units.
Tristable switching of a planar-aligned CSLC cell has been reported (I. Nishiyama et al. (1989) Jpn. J. App. Phy. 28:L2248; and A. D. I. Chandani et al. (1988) Jpn. J. App. Phy. 27:L729. The observed switching has a dc threshold and a hysteresis of the threshold voltage. The third state of such tristable cells has been linked with the presence of an antiferroelectric phase, designated SmCA*. This type of CSLC cell has been designated an antiferroelectric LC cell. The antiferroelectric phase can, for example, be generated in a SmC* material by application of an ac field across the planar-aligned liquid crystal. It has been suggested that the applied ac field induces molecular rearrangement in which the LC molecules are aligned via the large spontaneous polarization of the material. CSLC materials which can exhibit this antiferroelectric effect have been reported by K. Furukawa et al. (1988) Ferroelectrics 85:63; M. Johno et al. (1989) Jpn. J. App. Phy. 28:L119 and Y. Suzuki et al. (1989) Liq. Cryst. 6:167.
Lagerwall et al. in U.S. Pat. No. 4,838,663, describe a non-tilted, non-ferroelectric, chiral smectic A* (SmA*) liquid crystal electro-optic switch. With planar-aligned, surface-stabilized SmA* material between substrate walls with no electric field applied (zero field state), n is parallel to z (i.e., .PSI.=0.degree.). The molecular director of the SmA* material exhibits rotation in a plane relative to z (.PSI..noteq.0.degree.) in response to an applied electric field due to the electroclinic effect (first described by S. Garaff and R. B. Meyer (1977) Phys. Rev. Letts. 38:848). These cells display an analog dependence of .PSI. with applied field to a maximum tilt angle .PSI..sub.MAX, which angle is an intrinsic property of the SmA* material. Materials having .PSI..sub.MAX ranging from about 6.degree. to 22.5.degree. have been observed. The advantage of these planar-aligned SmA* cells is submicrosecond switching speeds and analog rotation of the optic axis.
A single SmA* cell between crossed or parallel polarizers yields less than optimal contrast or modulation depth due to the presently available small electroclinic tilt angle. The dynamic range of such a light switch can be improved by incorporating a series of two or more SmA* half-wave plates between crossed polarizers with sequential SmA* cells tilting in opposite directions from one another. Unfortunately, the improved dynamic range is often accomplished at the expense of cost, device simplicity and ease of operation. See also Sharp, G. D. et al., Optics Letters (1990) pp. 523-525.
L. A. Beresnev et al., European Patent Application No. 309774, published 1989, has recently described a new type of chiral smectic ferroelectric liquid crystal cell called the distorted helix ferroelectric (DHF), liquid crystal cell. This type of device is similar to the planar-aligned chiral SmA* device of Lagerwall et al., except that it is not strongly surface-stabilized, so that the helix along the direction of the layer normal, z is not suppressed. When the pitch of the helix (defined as the distance between identical orientations of n along the helix) is much shorter than the wavelength or wavelengths of light incident upon the device, light traversing the material sees an index of refraction given by the average orientation of the molecular directors. Application of an applied electric field to the DHF cell perpendicular to z, partially orients the molecular directors by an angle .PSI. to z. The angle .PSI. is dependent on the size and magnitude of the field so the DHF device operates in an analog mode similar to a SmA* device. In a DHF device there is a change in the birefringence of the material as the molecules align, which does not occur in either the SSFLC SmC* or planar-aligned SmA* device. The DHF materials, such as Hoffmann-La Roche DHF 6300, having .PSI..sub.MAX as large as .+-.37.degree. have been described. The advantage of DHF switching devices over other FLC switching devices described above is the variable birefringence with applied voltage. This is similar to the operation of nematic liquid crystals, which also yield a variable birefringence with applied voltage. In contrast to nematic liquid crystals, the DHF molecular directors rotate by their full tilt angle within 40 .mu.sec, a significant advantage. Furthermore, the voltages required to rotate the optic axis are generally much lower than those required for SmA* and SmC* cells. An interesting feature of DHF devices is the coupling of the change in birefringence with the rotation of the optic axis as a function of applied voltage.
Z. M. Brodzeli et al. (1990) Technical Digest on SLM's and Their Applications 14:128 have reported fast electro-optic response (20 .mu.sec) in a homeotropically-aligned SmC* liquid crystal. In homeotropic alignment, the smectic layers of the liquid crystal are parallel to the surfaces of the substrate walls (see FIG. 2) and as in planar-aligned CSLCs, the molecular director makes an angle, .PSI., with the layer normal. In the optical modulator described by Brodzeli et al., the homeotropically-aligned SmC* material is positioned between substrate walls having deposited electrodes (the width of the cell was given as 17 .mu.m.) Polarized non-monochromatic light entering the device, propagating along the axis normal to the layers, was reported to be modulated in intensity by application of a voltage across the electrodes.
J. Y. Liu et al. (1990) Optics Letts 15(5):267 report second-harmonic generation in homeotropically-aligned SmC* LC cells across which a dc electric field of 1 kV was applied.
Phase modulation of optical signals is often accomplished by means of an electro-optic effect in which a change in index of refraction of a suitable material is achieved with the application of an electric field. With crystalline materials exhibiting second-order optical nonlinearity, the electric field changes the index of refraction of such materials when seen by the incident optical field. The Pockel's effect refers to the linear variation of birefringence of non-centrosymmetric materials (including KD*P and LiNbO.sub.3) as a function of an applied electric field. The electric field perturbs the electron energy function, thus changing the index of refraction by approximately 0.01-0.001. There are two common Pockel cell modulators, referred to as transverse and longitudinal, depending on whether the applied electric field is perpendicular or parallel to the direction of light propagation. The related Kerr effect generally induces a birefringence in proportion to the length of the material, and the square of the electric field in isotropic materials. For both of these effects, it is an electronic excitation of the material that interacts with incident light.
Electroabsorptive phenomena such as the Franz-Keldysch and the Quantum Confined Stark effects can be also be used to modulate light. In the former, the absorption spectrum of a III-V material such as GaAs/GaAIAs is broadened by the application of an applied electric field (An example is the self-electro-optic effect device described in D. A. B. Miller, U.S. Pat. No. 4,546,244, and in the latter, the absorption edge is shifted by an applied electric field.
While the Pockel's and Kerr effects are high speed effects, they require large voltages, for bulk implementations in order to achieve very small electro-optic effects. For example, a half-wave of phase shift resulting from these electro-optic effects usually requires voltages on the order of kilovolts in electro-optic crystals such as LiNbO.sub.3, LiTaO.sub.2, KTP, etc. See, e.g., Yariv, A. and Yeh, P. Optical Waves in Crystals (1983) Wiley and Sons, New York Electroabsorptive modulators are also high speed, but suffer from limited throughput, and low contrast ratio.
A technique that has been used to improve the characteristics of electro-optic Pockel's and Kerr effect phase modulators, is to fold the optical path length using a Fabry-Perot etalon or resonator. This technique relies on interference of waves within the cavity and requires a lower voltage to achieve a desired phase shift compared to non-etalon electro-optic modulators. By virtue of the non-linear intensity transmission function of the Fabry-Perot resonator, a small induced phase change in turn produces a large intensity modulation. In other words, a Fabry-Perot etalon transforms the low amplitude input signal to an output optical intensity with high contrast. See Yariv, A. and Yeh, P. (1984) Optical Waves in Crystals, Chapter 8, John Wiley and Sons, New York.
This method can be used, with the resonance cavity operating in transmission mode or reflection mode, to obtain a large modulation depth with only a small electro-optically induced phase shift. However, because the electro-optic effect is very small, the cavity finesse must be very large to achieve a large optical modulation. A draw back of high cavity finesse, on the other hand, is low optical throughput.
A Fabry-Perot device consists of two plane parallel, highly reflecting surfaces, or mirrors, separated by a distance, L. When the mirrors are fixed at distance L, the device is called an etalon. When L can be varied the device is called an interferometer. A Fabry-Perot etalon operates on the principle of multiple interference of the waves reflected or transmitted by the mirrors. If L is a multiple of .pi., then the transmitted waves destructively interfere and the light incident upon the device is ideally totally reflected by the etalon. If L is a multiple of 2.pi., then the waves reflected by the mirrors cancel, and all the light is ideally transmitted by the etalon (assuming no absorption losses). If the etalon thickness is somewhere in between .pi. and 2.pi., then partial transmission or reflection occurs. If the optical thickness of the etalon can be changed, the etalon operates as a variable modulator.
Miller et al., U.S. Pat. No. 4,790,643, disclose an optically bistable device comprising a Fabry-Perot etalon containing an intracavity, optically non-linear, nematic liquid crystal material. The device provides an electro-optic bistable switch with possible applications in digital code modulation upon a laser beam. This device, however, has limited applications in that it is designed to modulate a monochromatic or coherent light source. The disclosed device is reportedly capable of tuning the etalon fringe electro-optically to a design wavelength such that a low intensity, coherent incident beam of a wavelength in proximity of the design wavelength induces bistable switching of the etalon. Since the liquid crystal of this device is neither chiral or ferroelectric, the switching speed of this particular optical modulator is relatively slow.